A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In conventional optical lithography, the pattern may be transferred from the patterning device to the substrate using a projection system comprising a series of lenses. The lenses may be transmissive or reflective, the choice between transmissive or reflective lenses generally being based upon properties of the wavelength of radiation beam used. The pattern projected onto the substrate may suffer uniformity degradation due to flare caused by lenses of the projection system.
Further, the resolution achievable using a given lithographic apparatus is dependent upon the wavelength of radiation that the lithographic apparatus uses. In recent years lithographic apparatus manufacturers have been working on developing lithographic apparatus which use wavelengths which are significantly shorter than the wavelengths currently used. These wavelengths are commonly referred to as extreme ultra-violet (EUV), and a relevant wavelength is typically at or around 13.5 nanometers. EUV radiation is absorbed by most materials, and this means that it is desired not to use transmissive lenses in EUV lithographic apparatus. For this reason, the projection system of an EUV lithographic apparatus is usually formed from a series of mirrors.
Users of lithographic apparatus are becoming increasingly interested in being able to control parameters of the radiation beam which is transferred by the projection system. One such property relates to the divergence of the radiation beam, and is commonly referred to as sigma (σ).